1. Field of the Invention
This invention relates to cutting tools and more particularly to use of plastic materials for fabrication of cutting tools adapted to cut circular holes or bores.
2. Background Information
Metallic materials are commonly used to fabricate the main bodies of cutting tools. For example, steel is typically used to fabricate tubes and discs to serve as the main bodies of core bits and circular or disc-shaped saw blades, respectively. Cutting elements, such as abrasive elements or cutting teeth are brazed, laser welded, mechanically fastened to, or formed integrally with, the steel core. Such steel cores operate satisfactorily in a wide range of applications. However, they are not without drawbacks. In particular, metallic cores are relatively heavy, thus tending to make such cutting tools cumbersome and difficult to handle in certain applications. Metallic cores also disadvantageously tend to vibrate and generate noise during cutting operations. Moreover, metallic cores are relatively expensive and comprise a significant portion of the overall cost of the cutting tool.
Some of these problems have been recognized and attempts made to address them with respect to disc-shaped cutting tools. For example, U.S. Pat. Nos. 5,408,983 and 5,411,010 both disclose circular saw blades and/or cutting discs that utilize reinforced plastic composites in the disc-shaped main body portion thereof. These configurations may provide such advantages as reduced tool weight and noise reduction.
Use of similar materials, and the advantages associated therewith, have not, however, been carried over to cylindrical core bit type cutting tools. This is presumably due to the significant distinctions between core bit and circular disc cutting tools and the cutting applications for which they are used. Indeed, it is generally understood by those skilled in the art that these two types of elements operate in distinctly different cutting regimes, each with a unique set of parameters that are not transferable therebetween. In this regard, data and methodology such as techniques, guidelines and accepted practices in terms of cutting rates, materials and cutting speeds associated with conventional disc-shaped cutting tools are generally inapplicable to cylindrical core bit-type cutting tools.
As an example of these differences, peripheral speeds at which each individual cutting tip or tooth moves through the workpiece, are vastly different. For instance, conventional diamond tipped circular (disc) blades commonly used for cutting hard materials, such as, for example, concrete, typically range from approximately 4 inches (102 mm) to 48 inches (1219 mm) in diameter. Conventional recommended operational speeds in rotations per minute (rpm) for these blades yield a preferred peripheral speed of approximately 49 meters per second (m/s).
On the other hand, diamond segmented core bits commonly utilized for cutting similar material (concrete), range in diameter from approximately 0.4 inches to 10 inches (10 mm to 250 mm) and may be as large as 36 inches (900 mm) or more in some applications. Recommended operational speeds in rpms yield a recommended peripheral speed of approximately 2.5 m/s. Such a wide discrepancy in peripheral speed, of more than an order of magnitude, is indicative of the non-analogous nature of these two distinct cutting tool types. Similar discrepancies in peripheral speed pertain to various cutting applications and workpiece materials, including, for example, asphalt, stone, reinforced concrete, limestone, silicon quartz, glass, etc.
Another factor that tends to militate against the use of plastic materials in core-bit applications is the relatively abrasive environment encountered by the body of the bit, relative to bodies of circular disc type blades, due to prolonged contact with grinding swarf. In this regard, during each revolution of a conventional disc type blade, an individual tooth or diamond enters the workpiece, removes some of the workpiece to thereby cut a kerf therein, then exits the workpiece. The removed material, along with any cutting lubricant or coolant, forms a relatively abrasive grinding swarf that is effectively carried through the kerf by the tooth and expelled therefrom as the tooth exits the kerf. In this manner, the swarf is effectively removed from the kerf (and from contact with the cutting tool) nominally as fast as the swarf is produced. The abrasive grinding swarf thus tends to contact only the teeth or cutting elements, rather than the relatively abrasion sensitive plastic composite body portion of the disc.
In contrast, the nature of core drilling applications requires the cutting teeth or diamonds to remain in the kerf until the cutting operation is complete. Accordingly, during most conventional cutting operations, the swarf, having no other space to occupy, tends to remain in the kerf where it climbs up the tube, carried by cutting liquid, as cutting progresses. Thus, in such cutting applications, the swarf remains in contact with the body of the core bit throughout the cutting operation. The deeper the cut or kerf, the greater the area of contact with the core body. This prolonged contact of the abrasive swarf with the cutting tool presents relatively aggressive working conditions to any components thereof fabricated from relatively soft and abrasion sensitive plastic materials.
Another aspect of core drill operation is that all cutting elements continuously cut the workpiece during operation. The cutting elements of a saw blade cut in an intermittent, interrupted fashion as each one of the elements rotates in and out of contact with the workpiece. Thus, the thermal stresses are higher in core drill operation than in saw blade operation.
Therefore, it is unexpected that a core drill bit made with a plastic core body can be used to cut concrete and other hard materials. It is even more unexpected that such a tool would have a useful life at least as long as a comparable tool made with a steel core.